The objective is to develop knowledge on the relation between ultrastructural change and physiological regulation of function. The function to be studied is excitation contraction coupling in crayfish single muscle fibers. Our specific aims are: to determine the identity and fluxes of the substances which change concentration in the myoplasm and cause a decrease in actomyosin ATPase activity inhibition. To these concentration changes with structural changes in the transverse tubular system and the sarcoplasmic reticulum. To quantitatively determine the regulation of ATPase rates by the load on the muscle, and to relate the rates to the heat of shortening. To study the effect of myofilament lattice configuration and volume on the ATPase rates and mechanical efficiencies.